Automatic impedance measuring device



Sept 11, 1956 R. E. HENNING Y AUTOMATIC IMPEDANCE MEASURING DEVICE FiledMarch 9,` 1953 km. mm.

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United States Patent O AUTOMATIC IMPEDANCE MEASURING DEVICE Rudolf E.Henning, New York, N. Y., assignor to Sperry 'Rand Corporation, acorporation of Delaware Application March 9, 1953, Serial No. 341,027

-'17 Claims. (Cl. 324-58) This invention relates to measuring apparatususeful in the microwave region of the frequency spectrum, and moreparticularly, is concerned with means for rapidly and automaticallymeasuring the reflection coellcient or impedance of a microwave load asa function of frequency.

The method in general use for making impedance measurements in themicrowave region is to utilize a slotted section of transmission linefor investigating standing waves along the line by means of a probemovable along the slot. information which can be obtained from the probeincludes the positions of the nodal points and the ratio of maximum tominimum voltage along the line. From this information, it is possible tocompute the normalized impedance in terms of the reflection coefficientat any point along the line looking toward the load. To obtain a plot ofimpedance at a function of input frequency, it is necessary that aseries of point-by-point computations be made, so that this method ofinvestigating the impedance characteristics of a load as a function offrequency is a slow and tedious process.

Some attempts have been made heretofore to make measurements ofimpedance by means of fixed probes spaced along the transmission linesuch as that disclosed in the copending application Serial No. 293,698,led June 14, 1952, now Patent No. 2,680,837. Another method is disclosedin Patent No. 2,605,323 to A. L. Samuel. In the Samuel method, fourfixed probes equally spaced at intervals of one-eighth wavelength alongthe line sample the waves existing in the transmission line thatconnects the test load to a microwave source. These probes are connectedin pairs, the two probes of each pair being spaced along the line by aquarter wavelength. The output of each probe is connected to asquare-law detector, such as a crystal diode, and the difference inpotential between the detector signals from one pair of probes, i. e.,the first and third probes in the order of position from the load, isapplied across the vertical deflection plates of acathode rayoscilloscope, while the difference in potential between the detectorsignals from the remaining pair of probes, i. e., the second and fourthprobes in the order of their .position from the load, is appliedacrossthe horizontal deflection plates of the oscilloscope.

Samuel teaches that the polar coordinates of the resulting spot on thescreen of the oscilloscope at any one frequency correspond to themagnitude and phase angle of the reflection coefficient of the load atthat frequency at a plane of reference a distance of one-eighthwavelength from the first probe. He further teaches that if the input tothe line is varied in frequency over a limited range, the oscilloscopespot traces out a path representing the desired curve on the reflectioncoefficient plane, giving a continuous polar coordinate plot of thereflection coefiicient of the test load as it varies with frequency.

While such a system provides a rapid method of indicating the change ofimpedance with frequency, in practice it is not sufficiently accurate toprovide a quantitative measurement of impedance except over an extremely2,762,972 Patented Sept. 11, 1956 limited bandwidth of operation. Thefixed spacing of the probes makes the system critical as to frequency sothat even over a 10% bandwidth of operation described as the usefulfrequency range of the system, the Samuel method of measuring impedanceis of little value other than as a qualitative check of the change inimpedance with frequency.

lt is the general object of this invention to avoid and overcome theforegoing and other difficulties of and objections to the prior artpractices by the provision of apparatus for measuring impedance in theultra-high-frequency region which is automatic, rapid, accurate, and isoperable continuously over a broad frequency band.

Another object of this invention is to provide means for producing avisual ind-ication of the reflection coefficient of a test load as afunction of frequency over a 40% frequency band without the necessity ofre-tuning, and with an accuracy such that at any given frequency withinthe band the impedance points in the reflection coefficient plane arelocated within a circle having the true impedance point at its centerand having a diameter of the order of 0.05 times the radius of the unitcircle, which is the radius corresponding to the maximum amplitude ofthe reflection coefficient such as produced by a short circuittermination.

These and other objects of the invention which will become apparent asthe description proceeds are achieved by the provision of apparatuscomprising a Variable fre'- quency microwave energy source, the outputfrequency of which may be varied at a linear rate over the operatingfrequency range. A main wave guide terminated in an energy-absorbingnon-reflecting impedance is coupled to the output of the microwavesource. First and second auxiliary slotted wave guide sections areconnected in parallel positional relationship, directional couplingmeans being provided to couple the incident energy in the main waveguide to the auxiliary wave guide sections. The first auixiliary waveguide section is terminated in a short circuit. The second auxiliarywave guide section is terminated with the test load.

Means including a pair of spaced probes extending through the slot inthe first auxiliary waveguide detect variations in the electric lleldalong the wave guide section. Servo means responsive to the differencein potential of the two probes moves the probe means in response to thedifference in potential of the two probes so as to maintain the probemeans at a point along the line Where the probes are at equal potential,whereby the distance of the probe means from the short-circuited end ismaintained proportional to the guide wavelength. Four movable probesextending through the slot in the second auxiliary wave guide sectionare linked together by means for moving each of the probes along theslot and for maintaining equal spacing among the probes. The means formoving each of the probes is actuated by movement of the probe meansalong the first auxiliary wave guide section, whereby the spacingbetween the four probes varies (with movement of the probe means alongthe rst wave guide section) in direct proportion to the guide wavelengthof the microwave energy, the spacing among the four probes beingmaintained at an eighth Wavelength or odd multiple thereof over theentire frequency range. The differences in the detected Voltage betweenthe first and third probes and between the second and fourth probes areindicated, recorded, or preferably plotted in rectangular coordinates toprovide information of the reflection coetiicient of the test load.

For a better understanding of the invention, reference should be had tothe accompanying drawing wherein:

Fig. 1 is a schematic diagram showing a preferred .Y embodiment of thepresent invention; and

'Fig Zshows an alternative means of recording the impedance information.

In the drawing, the numeral l) indicates generally a variable frequencymicrowave energy sourcewhich includes'atunableklystron or other highfrequency generala'tor. The 'microwave source luis mechanically tuned'overafrequency range, for example, of the order of 4000 'to6000megacycles, by any suitable drive means, auch asja motor`'2`an`daspeed-reduction gear Idrive 14. The mechanical Atuni'ng system of thesource`1`0 is preferably designed 4to provide asubstantially linearchange infrequency throughout 'the frequency range of the microwaveisorce.

The microwave source 10 is preferably vof Va"`s"ervo regulated'typeasdescribed in the copeiiding'application "SLN, 256,'S6ftiledfNovember'l?, 1951 in 'filename of Seymour Cohn. 'The' type of microwavesource there tdescijibtl hasafpuls'eltnodulated microwave output signal'in whiehfthef'fundamental harmonic content of thevpu'lse modulationsignal iis' maintained substantially contant by 'aifs'ervomcontrolcircuit, indicated at i6 in the drawing, varies the modulationpulsewidth in response to "any'fdtected vvariations inV the `amplitudeofthe funda- Imental harmonic.

'The pulsej'modulated output signal of' the microwavefsource"f10"`is'ooupledftoa hollow wave guide section 18 Whichends -inasuitablenonreecting termination 20. fpair of 'axiliarfwaveguide"sections'22 Vand 24 are eoupledto the main Wave guide section i8 by cross-guide `tliee'tip nal coupler means 26 and '28 respectively.'Snit- `able c rossguide couplers are described in'Patcnt No.2,602,859L`t T. Moreno. Each coupler includes a pair ofc'rossishapedape'rtures`30 positioned along a diagonal inthe common Wallportionbetween the main wave guide andauxiliary' waveguide section, asshown in the drawiha..

'Both'of the'V auxiliary 'wave guide sections 22 and 24includeolottedline portions, 'the slots being indicatedl at '32andf`34respectively. The rst auxiliary Wave guide section22'isterminated in ashort circuit provided'bya conducting plate"36, theshort-circuiting'termination producing standing Wayes along the firstauxiliary waveguide secition'22. "Thereflected energy is absorbed by anonreiiecting termination 33 at theoth'er end of the auxiliary wave.'guide section 22. 'opposite theV 'short-circuiting plate 3 6. Thesecondfauxiliarywave guidesection 24 isY Vterminated'in a ftestioad,indicated vat 3 5, which is coupled tojtheL en dofjthe wave guidesection 24 by means of a liange 31' toyvhic'hv the test`load is'secured,f'as by bolts '39. Vv"I`l'1 e' en e'rgyreected by the test loadAis absorbed by a o' reecting l-terminati'c'nn'41 at the opposite end ofthe 'guide Seton-.24. Toobtaininformation' as to thev Wavelength of'energy in thewavejguide' s'ection'22, a servo operated probe'system"`is^p`r oy ided'which includes a probe carriage 38movablefalongrtheslot 32. Theprobe carriage 38 issuppoitedforf'movement'along the slot 32 by means of a supportingi'arm#B which threadedly engages a screwfeed shaft "'42the'sh'aft'being'journalled at its ends in suitable bearing plates"'44"andf46 rigidly supported be'tivveen' the auxiliarywave guidesections22 and'24. Movementof l'the'probe carriage" 38 'is etfected by means'ofa vservo motor '48 whi'chdrives theshaft 42th'rough'a suitable speedreducer 3'5'0.

Supported in spacedl relationship by the-carriage 38 are. a pair vofprobes 52 and S4 which extend into the interior' of the wave guidesection 22. For'best operation of '.t-hese'rvorprobe system, the spacebetween the' probes shouldbe o f' the order of a quarter Wavelength asmeasured at the mid-band frequencyof the operating frequency range. Athird probe 56 is supported bythe probe carriage 38 halfvtfay betweenthe probes 52 and 54, voltage at the prob e 56 -being coupled to theservo control 16 through a suitable detector 57 to provide the necessarysignal indicative of variations in the amplitude of the fundamentalharmonic of the pulse modulation signal required for regulating thesource 10. The c'enterprobc 56 is normally positioned at the irstvoltage maximum, a quarter wavelength from the short-circuiting plate36. To maintain the center of the probe carriage 38 at a distance of aquarter wavelength from the short-circuiting plate 36 as the microwavesource 10 sweeps through the entire frequency rangejtheprobes 52 and 54are connectedthrough crystal diode detectors`58 and`60toi'espectivetunedaudio amplifiers 62 and 64. The latter are y'tuned to the pulse:modulation 'frequency V``f"`tl:le modulated 'microwave signal. A'p'airof `diode`rectiers 66 and- 68l connect the loutput. signalsofthearnpliers 62 and 64 across a filter network including a resistor 70and capacitor 72. The rectifiers are' connecting in opposingrelationship so as to conduct current through the resistor 70 inopposite directions. The opposed rectiers thereby produce a D.C. voltageacross the resistor 7 0 which is proportional to the difference invamplitudeof the 'output signals from the amplifiersZ and 64.

k'It'wlill be evident that the polarity ofthe `DJ-C. voltage producedacross :the resistor 70 'reverses'as' the amplitudes 'of therespfectivesignals detected by the probes52'a`nd S4 pass through a condition ofequal amplitudes. "lf'i't'he probe'V 'carriage 3S is centered ata'voltage maximum in the' standing 'wavesalong the line 22, the probes52 and 54, 'being symmetrically positioned 'about the maximum, detectsignals of equal amplitudes, and the'resultingpotential across the'resistor 70 iszero. As the frequency 'changes andthe standing wave-patternshifts along the line, :there is a momentary increase of signal'str'engthat one ofthey probes and'a decrease -of 'signal strength atthe otherprdbeproducing a momentary D.C.` error'voltage across theresistor' 70.

A T he'a'rrnature (notshown) of the separately-excited ield-typeinotor48 isV connected vto the output circuit of a D.C.'pow'er amplifier 76which receives its input voltage across the resistor '70,. the directionof rotation of the motor 48 being `thus controlledin response to thepolarity of 'the D.-CJ'potential acrossV the resistor 70. Rotationof'themotor d inresponse to the error voltage across the resistor'"7tr'esults`in a repositioning of the probecarria'ge 3S in'adire'ctionto minimize the error voltage and 'thus maintain lthe Lprobecarriage at the voltage maximum in the standing waves. Thus, the probecarriage is' made to track'the voltage maximum as 'it moves along 'theline with 'changing frequency.

"Although a 'Azero difference signal between'the probes occursutfhen`"the carriage 38 'is 'positioned symmetrically with respect; to eitheravoltagemaximumor minimum, at only one ofthese'points, namely a voltagemaximum, is thesei'fo operated'probesystem in astajble balancedcondition; ias determined by the direction'of rotation" of `xtheni'otor'with a qgiv'enpolari'ty'of voltage across theresistor 70. It'Will 'bes'een 'from 'the above description that the distance'ofthefcentorof'theprobe' carriage 38' from the short-cicuiting plate' 36 isvmaintained 'proportional 'to' the guidetwat'eleng'th'through the entirefrequency range of the"`r'nicrowavejsignal.

To nieasure'the ree'ction oo eicient o f the test load 35, fourprobesgindicated at 7S, 80, 82,' and' 84 'are 'positicned' along theslot 34`of the auxiliarywave guide section 24. The 'probelis 'supportedfor movement along the/slot 34`byzmea1sofan extension 'of thevsupporting arm i'and is maintained a' quarter'wavelengthv away 'fromthe junction :ofthe test load'and the 'end of the auxiliary waveguide"section 34'iwith movement of the supporting arm' `40'by` ytheservo'operated probe systm'above de- A scribed. i Theend of theauxiliary wave guide-section '24 is 'preferably' inthe same plane as theshort-circuiting termination 3 6.

'l`.l 1e probe 7S-.is supportedV for movement lalong the slot 34 ,bymeans of a' supporting arm -86 which th-reade'dly engagesv ascrew-feedshaft1 87,l the shaft being journalled afits endsin'tbebearingplates '44 -and 546. Y Simi-larlyg-the probe 82 issupported for movement along the slot 34 by means of a supporting arm 88which threadedly engages a screw-feed shaft 89, and the probe 84 issupported by means of a supporting arm 90 which threadedly engages ascrew-feed shaft 91. The probes are maintained in spaced relationship atequal intervals of an eighth wavelength by gearing each of the shafts87, 89, and 91 to the shaft 42. Spur gears 92 and 94 rotate the shaft 87one revolution for each two revolutions of the shaft 42, while spurgears 96, 98, and 100 rotate the shafts 89 and 91 from the shaft 42 atone and one-half revolutions and two revolutions respectively for eachrevolution of the shaft 42. Thus, it will be seen, assuming an equalscrew pitch on each of the shafts 42, 87, 89, and 91, that the fourprobes along the slot 34 are maintained at equal spacing of an eighthwavelength over the entire frequency range of the microwave source bythe servo operated probe system in the auxiliary wave guide section 22.

Suitable square-law detectors, such as crystal diodes or barretterwires, indicated at 102, 104, 106, and 108, couple the probes 78, 80,82, and 84 to tuned audio arnplifiers 110, 112, 114, and 116respectively, the amplifiers being tuned to the modulation frequency ofthe microwave energy. Alternate amplifiers 110 and 114 are connected byopposed rectifiers 118 and 120 to a filter network including a resistor122 and capacitor 124, the voltage across the filter network beingproportional to the difference in signal strength at the output of therespective detectors 102 and 106. A zero centered D.-C. voltmeter 125indicates the variations in potential across the resistor 122.Similarly, the outputs of the amplifiers 112 and 116 are connectedthrough opposed rectifiers 128 and 130 across a filter network includinga resistor 132 and capacitor 134. The voltage appearing across thefilter network 132, 134, which is proportional to the difference insignal strengths at the output of the respective detectors 104 and 108,is indicated on a second zero centered D.C. voltmeter 135.

The reflection coefiicient i; of the load 35, which is a complexquantity, may be represented in the form lZ-:preal-I-pim If the probesare spaced at eighth wavelength intervals along the guide from the testload, the difference in potential between the square of the voltages atthe first and third probes in their orders from the test load isproportional to the imaginary component pim of the refiectioncoefiicient. The difference in potential between the square of thevoltages at the second and fourth probes in turn is proportional to thereal component pren of the reection coefficient. Once the voltmeters 125and 135 are calibrated they indicate directly the real and imaginarycomponents of the reflection coefficient of the test load at thefrequency of the microwave signal at that instant. The frequency sweepof the source must be made quite slow, or the frequency manually changedin steps, however, to take advantage of the voltmeters in getting aquantitative measure of the refiection coefcient or impedance of theload as a function of frequency.

A more convenient method of presenting the measured refiectioncoefficient information is to plot the real and imaginary components ofthe reflection coefcient in rectangular coordinates. This may beaccomplished by suitable rectangular coordinate plotting means asindicated at 136 in Fig. 1. Plotting means 136 includes a heated tracingelement 138 which is supported by a laterally moving supporting member139. The tracing element 13S is in contact with the surface of a chart140, the chart being coated or impregnated with a material that producesa stain when heated, whereby the heated tracing element 138 produces acontinuous line on the chart along the path of movement of the tracingelement 138. Such thermally sensitive chart papers are well known andcommercially availaible. However, any suitable recording medium may beused, such as a recording pen and ink.

The chart is mounted on a movable belt 142 which extends betweenrotatable rollers 144 and 146. It will be seen that rotation of therollers effects movement of the chart 140 along one coordinate relativeto the tracing element 138. Rotation of the roller 144 is provided by aservomotor 148 suitably geared to the shaft of the roller 144.

To effect relative movement between the tracing element 138 and thechart 140 along the one coordinate in response to the real component ofthe reflection coefficient of the test load 35, the angular position ofthe servomotor 148 is controlled in response to the variations involtage across the resistor 132. This is accomplished by comparing thevoltage across the resistor 132 with the voltage across a bridge circuit152, which includes a potentiometer 154 connected across a D.-C.potential source 156 and mechanically coupled to the servomotor 148 forrotation of the sliding contact. The difference in potential across theresistor 132 and across the bridge circuit 152, as measured between thesliding contact of the potentiometer 154 and the center point of theshunt resistor 158, is amplified by a suitable D.C. amplifier 160 andapplied to the servomotor 148. Rotation of the servomotor 148 inresponse to the output signal from the amplifier 160 rotates the slidingcontact of the potentiometer 154 until the input signal to the amplifier160 is reduced to zero. Thus, the angular position of the output shaftof the servomotor 148 is maintained proportional to the voltage acrossthe resistor 132, the position of the tracing element 138 relative tothe chart 140 along one coordinate being thereby maintained proportionalto the voltage across resistor 132.

Movement of the tracing element 138 relative to the chart 140 in theother rectangular coordinate is similarly controlled in response to theimaginary component of the reflection coefficient as indicated by thevoltage across the resistor 122. This is accomplished by means of aservomotor 162 which is geared to a screw-feed shaft 164 that threadedlyengages the supporting member 139. The angular position of the outputshaft of the servomotor 162 is maintained proportional to the voltageacross the resistor 122 by comparing that voltage with the voltageacross the bridge circuit 164. The latter circuit includes apotentiometer 166 mechanically coupled to the servomotor 162. Anydifference in voltage across the resistor 122 and the bridge circuit 164is amplified by a suitable D.C. amplifier 168 and applied to theservomotor 162. Rotation of the potentiometer 166 is thereby effected soas to reduce the input to the amplifier 168 to zero and thereby maintainthe position of the tracing element 138 relative to the chart 140 alongthe other coordinate proportional to the voltage across the resistor122.

The drive connections to potentiometers 154 and 166 are indicatedschematically; each of these connections is in practice arranged toretain the potentiometer arm within its normal range of angular movementfor full-range operation of the recorder system.

The resulting trace on the chart 140 is a plot of the refiectioncoefiicient of the test load, as it varies with frequency, in thereflection coefficient plane, the polar coordinates of any given pointalong the trace represent ing the amplitude and phase of the reflectioncoefficient of the test load 35 at the corresponding frequency. Thecenter point of the rectangular coordinate plot and the origin of thepolar coordinate plot of the reection coefficient, which corresponds toa load impedance that is matched to the line and produces zeroreflection, can be established on the chart 140 by interrupting theoutput of the source 10. The maximum amplitude of the reflectioncoeflicient vector is unity, the unit circle on the chart 140 beingdetermined by substituting a short circuit termination for the test load35. The unit circle establishes a scale factor from which absolutevalues of the reflec- .arma-,97e

vtion `coelcient can bedetermined. To obtain :impedance informationdirectly, a printed Smith chart (described in Electronics, January 1939,`in the article Transmission iLine Calculation by P.V H. Smith) maybe-used V as the chart 140, the trace of the reflection coeiricientonthe Smith chart being read off directly in terms .of thenormalizedresistanees and reactances of the test load 35,.

*As .the variation in amplitude and phase of'the reflection coeliicientof the load is continuously recorded .on the chart '140, it is desirablethat some frequency :reference .be superimposed on .the chart so thatmeasurements can beinterpreted in terms of .the instantaneous frequencyof .the energy `transmitted to the load'under test. One wayofaccomplishing this quite simply is to provide-apswitch -170 which .is.mechanically actuated v byLthe frequency tunmeans of the microwavesource 10. The switch 170 .mayi-be actuated by'a cam .172 .driven fromthe speed reducer 1A. The switch .normally connects .a -small heatingcurrent from across a portion of a voltage source 174 .through.the-tracing element 138 to produce alight trace .on .theirthermally.sensitive .chart paper, but when momentarily actuated by the cam 172,connects the .tracling-element 138 across the full voltage of the.source 174. Reversing of ythe switchll thereby producesincreasedheating of theV tracing element 138, resulting in a: momentary .increasein the density and area of the trace on .the .chart paper. These spotsin the trace occur `at periodic xedfrequency intervals, such as every 50or l00megacycles change in frequency, as the motor '12 changes theoutput vfrequency of the source 110 through the range. Thus, a series ofspots aresuperimposed along the-trace on the 4chart which can becorrelatedto the changing microwave frequency.

An alternativemeans of recording the changes infload impedance :withfrequency is illustrated in `Fig. 2 and includesV a continuous striprecorder indicated generally at176. 1A paper strip l78`is fed past apair ofrecording elements 180 and 152 by mechanically driving the stripbymeans .of a sprocket wheel '184 rotated by the Vtuning motor-12through asuitable speed reducer 186. The recording elements.-are'movedlaterally across the strip in response .to the real andimaginary components of the reection `coeicient of the load asVindicated by the voltages across the resistors .122 and-132 (Fig. l)respectively. -This is accomplished by actuating the tracing elements180 and .182 by means of theabove-identifiedservomotors 148 hand '162,'as by means 'of screw-feed shafts 188 .and :190that .threadedly Vengagetracing elements 180 andlSZrespectively.

Thestrip .recorder E76 provides a pair of line traces, indicated at 192and 194, which'indi'cate variations in the real'and imaginary componentslof the reilectioncoecientgas a-function of the changing frequency ofthemicrowave source. This form of recorder information has theadvantagethatthe frequency kscale is stretched out and linear whereas intherectangular coordinate plot of .the .components .of the reflectioncoeicient, the `spacing .of .the frequency reference spots isnot linear.

To obtain accurate results with the apparatus abovedescribed, care mustbe exercised in matching the detectors '102, .104, 106, and 108 so theyhave thefsame sensitivity and follow the same law with changes in'signalstrengm. The detector 57 also must be matched to these fourdetectors sothat the control of the microwave energy over the frequency range willbe consistent. The ampliers .1110, 11'2,"1Ir4, and 116 `must bc matchedin gain. Probes ymust be used that give broadband performance, andlmust.not befso large Vas to distort the eids in the slotted wave guidesections. Other factors indesign will befapparent to one skilled in theart as requiring care in selectioniof components lto minimize`measurement errors. From the above description it will be recognizedthat the objects ofthe invention have been achieved by the provision lofmicrowave l impedance lmeasuring equipment utilizingxthel. method :inwhich probesrspaced along vthe 'd transmission .line-to the:testdoadprovide .information plotted as1ga ,trace orftraces indicative:of .the variation in amplitude and :phase-.of fthe reection coeeientcfthe load with changing'frequency. vThe spacing of the probes is adjustedto maintain ,a spacing of .an eighth wavelength by servo meansresponsive to `changes in guide "wavelength, resulting` in a moreaccurate Yimpedance measuring apparatus :and one which is operable overa greatly exparidad` frequency scale. Since the spacing between thesampling probes is adjusted with wavelength, theA intervals between theprobes need not be restricted to, an eighth wavelength, as is `necessarywith fixed .sampling probes vto obtain maximum-bandwidth of operation,but maybe spaced atintervals .of any oddmultiple of fan eighthwavelength. (It 'should be noted, however, `that with spacings at-multiplesof 3, 7, 11, of an eighth wavelength, the connections of theprobes 78 `and'Z Vwith the amplifiers and .114 `should beinterchangedfso that the probe 78 would be connected to the amplierllsiandthe probe 82 Wouldbe connected to theamplifier 110 in the diagram ofthe ligure.) Larger spacing between the probes is desirable where thewavelength becomes so short asto make -it mechanically impracticaltospace the probes at an eighth wavelength.

It should be noted vthat the invention contemplates what amountsineffect to a vector summation of the squared output signals from thesquare-law detectors, where Vthe vector Arepresentations of the outputsignals have lengths proportional to the magnitudes of the respectiveoutput signals, `andthe vectors are spaced at intervals of 90, i. e.,360 dividedby the number of probes. Taking the difference between thedetector outputs from alternate probes, therefore may equally -as wellbe considered as taking the sum of two vectorquantities at 180 relativetoeach other. lThe'inall resultant then is obtained-by taking theresultantof this-vector sum and the-resultant ofthe vectorV sum of 'theoutputs of-the other pair of alternate probes and adding themvectorially as two vectors at 90 relative to each other.

ISince-many changes Acould be madezinthe .above-'construction and lmanyapparentlywidely different .embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained :in the above description or .shown in the .accompanyingdrawingsshall beinterpreted vas illustrative and not in a limitingsense.

Whatis ,claimed is:

l. Apparatusfor Ymeasuring thexmpedance of a'microwave Itest lo.ad,saidapparatus comprising a variable ffrequency microwave `energy source,means for varyingthe signal frequency -ofgthe `source .over theoperatingfrequencyrange at .asubstantially linear rate, amainrwavevguidecoupledloithe output of the miercwaversourceand terminated -n anenergy-absorbing non-reflectingimpedancefrstand second .auxiliary wave,guide sectionssublstantially parallel'to :each 'other Vamd havinglongitudinal slots therein, `directional coupler means forcouplingrincident .energy in the'main wave guide into each of theauxiliary wave guide sections, kthe first auxiliarywave guide sectionbeingterminated in a short circuit andthe second auxiliary wave guidelsection being terminated. in the test load, means-,including a pairofuspaced probes .extending-r through the slot in the iirstauxiliarywave guide section for .detecting variations in the electric field alongthe wave .guide.section, motor means mechanically coupledtotheprobemeansfor imparting movementthereto along'the slot .in the lrst auxiliary waveguide section, servo controlmeans responsive to the difference inpotential of .the two probes for actuating the motor means to positionthe probe means at .a point where the probes areat equal potential,whereby the distance of the probe meansfrom the short-circuited end ofthe first auxiliary wave guide section is maintainedproportional to theguide wavelength, four movable Iprobes extending throughthe s'lot Ainthe-second auxiliarycwaveguidesectiong--means for moving each of theprobes along the slot, means for maintaining equal spacings among thefour probes and between the rst probe and a fixed reference point alongthe wave guide adjacent the load, said means for moving each of theprobes being actuated by the motor means whereby the spacing between anytwo successive probes along the second auxiliary wave guide sectionvaries with movement of the probe means along the first auxiliary waveguide section, the four probes being spaced at intervals of an eighthwavelength or odd multiple thereof, means for rectifying the outputsignals from the four probes, means for amplifying each of the rectifiedsignals, means for producing an output signal indicative of thedifference in amplitude of the rectified amplified signals from thefirst and third probes, means for producing an output signal indicativeof the dierence in amplitude of the rectified amplified signals from thesecond and fourth probes, rectangular coordinate plotting means, andfirst and second servomotor means for controlling respectively the twocoordinates of said plotting means, the first servomotor means beingresponsive to one of said difference signals and the second servomotormeans being responsive to the other of said difference signals.

2. Apparatus for measuring the impedance of a microwave test load, saidapparatus comprising a variable frequency microwave energy source, meansfor varying the signal frequency of the source over the operatingfrequency range at a substantially linear rate, a main wave guidecoupled to the output of the microwave source and terminated in anenergy-absorbing non-reiiecting impedance, first and second auxiliarywave guide sections substantially parallel to each other and havinglongitudinal slots therein, directional coupler means for couplingincident energy in the main wave guide into each of the auxiliary waveguide sections, the first auxiliary wave guide section being terminatedin a short circuit and the second auxiliary wave guide section beingterminated in the test load, means including a pair of spaced probesextending through the slot in the rst auxiliary wave guide section fordetecting variations in the electric field along the wave guide section,motor means mechanically coupled to the probe means for impartingmovement thereto along the slot in the first auxiliary wave guidesection, servo control means responsive to the difference in potentialof the two probes for actuating the motor means to position the probemeans at a point where the probes are at equal potential, whereby thedistance of the probe means from the shortcircuited end of the firstauxiliary wave guide section is maintained proportional to the guidewavelength, four movable probes extending through the slot in the secondauxiliary wave guide section, means for moving each of the probes alongthe slot, means for maintaining equal spacings among the four probes andbetween the first probe and a fixed reference point along the wave guideadjacent the load, said means for moving each of the probes beingactuated by the motor means whereby thel spacing between any twosuccessive probes along the second auxiliary wave guide section varieswith movement of the probe means along the first auxiliary wave guidesection, the four probes being spaced at intervals of an eighthwavelength or odd multiple thereof, means for rectifying the outputsignals from the four probes, means for amplifying each of the rectifiedsignals, means for producing an output signal indicative of thedifference in amplitude of the rectified amplified signals from thefirst and third probes, means for producing an output signal indicativeof the difference in amplitude of the rectified amplified signals fromthe second and fourth probes, and means for plotting variations in eachof the difference signals as a function of time.

3. Apparatus for measuring the impedance of a microwave test load, saidapparatus comprising a variable frequency microwave energy source, meansfor varying the signal frequency of the source over the operatingfrequency range at a substantially linear rate, a main wave guidecoupled to the output of the microwave source and terminated in anenergy-absorbing non-reflecting impedance, first and second auxiliaryWave guide sections substantially parallel to each other and havinglongitudinal slots therein, directional coupler means for couplingincident energy in the main wave guide into each of the auxiliary waveguide sections, the first auxiliary wave guide section being terminatedin a short circuit and the second auxiliary wave guide section beingterminated in the test load, means including a pair of spaced probesextending through the slot in the first auxiliary wave guide section fordetecting variations in the electric field along the wave guide section,motor means mechanically coupled to the probe means for impartingmovement thereto along the slot in the first auxiliary wave guidesection, servo control means responsive to the difference in potentialof the two probes for actuating the motor means to position the probemeans at a point where the probes are at equal potential, whereby thedistance of the probe means from the short-circuited end of the firstauxiliary wave guide section is maintained proportional to the guidewavelength, four movable probes extending through the slot in the secondauxiliary wave guide section, means for moving each of the probes alongthe slot, means for maintaining equal spacings among the four probes andbetween the first probe and a fixed reference point along the wave guideadjacent the load,;

said means for moving each Iof the probes being actuated by the motormeans whereby the spacing between any two successive probes along thesecond auxiliary wave guide section varies with movement of the probemeans along the first auxiliary wave guide section, the four probesbeing spaced at intervals of an eighth wavelength or odd multiplethereof, means for rectifying the output signals from the four probes,means for amplifying each of the rectified signals, means for producingan output signal indicative of the difference in amplitude of therectified amplified signals from the first and third probes, means forproducing an output signal indicative of the difference in amplitude ofthe rectified amplified signals from the second and fourth probes, andmeans responsive to each of the difference signals, said means includingmeans for indicating variations in amplitude of each of said differencesignals.

4. Apparatus for measuring the impedance of a microwave test load, saidapparatus comprising a variable frequency microwave energy source, meansfor varying the signal frequency of the source over the operatingfrequency range at a substantially linear rate, a main wave guidecoupled to the output of the microwave stource and terminated in anenergy-absorbing non-reflecting impedance, first and second auxiliarywave guide sections substantially parallel to each other and havinglongitudinal slots therein, directional coupler means for couplingincident energy in the main wave guide into each of the auxiliary waveguide sections, the first auxiliary wave guide section being terminatedin a short circuit and the second auxiliary wave guide section beingterminated in the test load, means including a pair of spaced probesextending through the slot in the first auxiliary wave guide section fordetecting variations in the electric field along the wave guide section,motor means mechanically coupled to the probe -means for impartingmovement thereto along the slot in the first auxiliary wave guidesection, servo control means responsive to the difference in potentialof the two probes for actuating the motor means to position the probemeans at a point where the probes are at equal potential, whereby thedistance of the probe means from the short-circuited end of the firstauxiliary wave guide section is maintained proportional to the guidewavelength, four movable probes extending through the slot in the secondauxiliary wave guide section, means for moving eachA ofthe-probes `alongthe Lslot, means formaintainingequal spacing among ,thel fourprohes.andzbetween vthe .first probe and. a .tixed reference pointi alongthevwaveguide adjacentthe load, said imeans for movingeachof .the probes:being actuated 'by the. motor-means whereby-.the spacing between anytwo successive;:probespalong the secondauxiliary wave guide. .section`.varies :with movement-.of the probe means `along the first -auxiliarywave guide section, the'fourprobeswbeingzspaced at intervals of -aneighth wavelength `or oddfmultipleethereof, detectors forrectitying .theoutput signals'fromtthe four probes, and means responsive to .the.voltage .diiierence between theoutput of the detectors Acoupled-to.theiirst and .third probes. .and .responsive .to .the voltagediierencezbetween the. .outputs .of Athedetectors vvcoupled .to theYsecond .and fourth-probes, .said .means including means :for`indicating variationsiin said v oltage diierences.

5. Apparatus '.fonmeasuring the impedance of a microwave; test load,said .apparatus comprising a variable frequency'rnicrowave energysource, first andsecond wave guidev sections havinglongitudinalV slotstherein, means for coupling incident 4energy from the energysource intoeachof the wave guide sections, the iirst wave guide section beingterminated in a short .circuit and the second wave guide .section beingterminated in the test load, means .including a pair of spaced probesextending through the slot in the first wave guide section for detectingvariations in the electric iield along ythe wave guide section, motormeans mechanically coupled :to the probe means for .imparting movement-thereto along the slotin the iirst wave guidesection,servoxcontrol-means responsive to the difference in potential ofthe twoprobes for actuating the motor means to position the probe means at apoint where the probes areat equal potential, whereby the .distance ofthe probe means from the short-circuited end of the irst f wave guidesection is maintained proportional to the guide wavelength, four movableprobes extending through the slot in the second wave guide section,means for moving each of the probes along the slot, means formaintaining equal spacings among the four probes .and betweenthe iirstprobe and a fixed reference point along the` wave guide adjacent theload, said means l`for moving each .of the .probes .being actuated bythemotor means .zwhereby the. spacing between .any two successive probesalong the second Wave guide section varieswith movement ofthe-probemeans along the rst waveV guidesection, the fourprobes-being spaced atintervals of an eighth wavelength or odd multiple thereof, detectors forrectifying the output` signals from the four probes andmeans-responsiverto the voltage difference between the outputs of thedetectors coupled to the iirst and third probes and responsiverto lthevoltage difierence between the outputs ofthedetectors coupled to thesecond and fourth probes, said `means including meansfor indicatingvariations intsaid voltage differences.

- 6. Apparatus as defined in claim 5 further including meansforrregulating thevpower output of the microwave energy source, a probeextending :throughthe'slot in the first wave guide section andpositioned intermediate said pairv of probes. said probe being'movablewith `saidprobe means, and means for detectingthe-output-signal at saidprobe, the output of the detecting means 'being Vcoupled tosaid.regulating means for controlling the output ofthe microwave source inresponse 'tovvariations --in -the output signal at said probe.

7. Apparatus .for measuring the impedance of a'microwave test'load,saidapparatus-comprising a variablefrequency` :microwave .energy source,lirst fan'd r second wave guide sections having longitudinal slotstherein, means for coupling incident'energy fromthe energy sourcefintoeach of the wave guide sections, the rstwave Lg-ui'de section beingterminated in` ashort circuit andthe second wave guide section beingterminated :inthe test load,probe means extending 4through :the slot iinthe'jirst 4wave# :guide section for .detecting variations-in the.electric tieldsalong the wave :guide section,.motor'mean smechanicallyzfcoupled to the probe means `for imparting movementttheretoalong/the .slot in the first waveguide section,.means `electricallycoupled to the probe means for .controllingthe motor means in responseto .changes in:the;standingzwave pattern along the irst wave guidesection as detectedyby the probes, four movable probes. extendingthroughtthe slotin the second wave guide section, meansfformoving eachof the probes alongthe slot, meansfor` maintaining equal .spacings amongthe four probes and betweensthe rst probe and a iixedy reference pointalong the-.waste guide adjacent the load, gsaid means formoving eachoftheprobes. being actuated by the motor meansivliereby the spacing.between any two successive probes .along-the second wave guide sectionvaries with .movement ofithe probe Vmeans along the iii-st waveguide.sectionpithesfour probes being spaced at intervals of anxeighthwavelength or odd multiple thereof, detectors .for -rectifyingthe outputsignal vfrom each of .the four probes,r:andmeans;re sponsive to thevoltage .difference between the .outputsfof the detectors coupled to-theiirstandthird probes 1and responsive to the voltagediierencezbetween the 'outputs of the detectors coupled to the secondand fourthiprobes, said means including means Yfor indicatingVariationsn said voltage dierences.

8 Apparatus fOr measuring the impedance oa-microwave test load, said`apparatus comprising .ayariablezfrequency microwave energy source,tirstlandsecondfwave guide sections having longitudinal; slotstherein,t;tmcans for .coupling incident energy fromtheAenergy..source,into each of the wave guide sections, theirstwaveguklec@ tion being terminated in la shortcircuitand the secondWave guide .section being terminated Ain Ihe..tcsltl0ad,..se1'vooperated probe means `.associated @with :the rstrwave guide section, theprobe meanszbeing .automatically positioned along the first waveguide:sectionjnA fixed. relation to the detected standing wavepatternjherealong,four movable probes extending'throughltheslotziintheSCGOnd Wave guide section, means for moving .cach ofthe-probes along theslot, means forl maintaining equalpacings among the four probes,saidmeans forfmovngeahzof the probes being actuated by .said :servooperatedfprobe means whereby the spacing between any two successiveprobes along the second wave gnidezsection varies-with movement of theprobe means alongtheiirstwavmguide section, the four probes .beingspaeedatt-.internals ofean eighth wavelength or odd multiple.thereof,ldetectlr.s for rectifying theoutput signals from-each of thefour-probes, and means Yresponsive to the vvoltage1 diiierence. betweenthe outputs of the detectors coupled. to the rstnndthird probes andlresponsive tothe voltage difference between the outputs of thedetectors coupled tothe second-and fourth probes, said means includingmeans-,forindicating variations in said voltageditferences.

9. Apparatus for measuring the impedance of :a-test load.said apparatuscomprising a variablerequencyrnicrowave source, ,means including-asectionfoslottcdtransmission line for coupling .energyfromvthesourcetothe testrload, tour movable probes positioned along-the slotted linesectiommeans for movingthe.probesfalollgithc slot, .said means includinglinkage V.means for maintaining the .ratios of. distances betweenadjacent probes and..l the distance between any .one probe ,andthen-test` loadteonstant with movement of .the probes, means responsive.to

change in wavelengthof the energy. transmttedftothe test load, meansAactuated by the wavelength responsive means for actuating theprobesmovingrneans withchange in wavelength, the said Vdistances betweenadjacent probes f and any one of the probes and thetest load "beingganeighth wavelength Yor odd multiple thereof, and means responsive to thedifference lin squares o f -the `voltages at the first and third -probesand yresponsive to the difference in thefsquaresof :the'voltages atthelsecondandfourth probes in their order from the 'test load,said-last-nauri'eil means including means for indicating variations inthe respective differences.

10. Apparatus for measuring the impedance of a test load, said apparatuscomprising a variable frequency microwave source, means including asection of slotted transmission line for coupling energy from the sourceto the test load, four movable probes positioned along the slotted linesection, means for moving the probes along the slot, said meansincluding linkage means for maintaining the ratios of distances betweenadjacent probes constant with movement of the probes, means responsiveto change in wavelength of the energy transmitted to the test load,means actuated by the wavelength responsive means for actuating theprobe moving means with change in wavelength, the said distances betweenadjacent probes being an eighth wavelength or odd multiple thereof, andmeans responsive to the difference in the squares of the voltages at thefirst and third of the probes and responsive to the difference in thesquares of the voltages at the second and fourth of the probes in theirorder from the test load, said last-named means including means forindicating variations in the respective diiferences.

11. Apparatus for measuring the impedance of a microwave load, saidapparatus comprising a source of microwave energy, means including asection of slotted line for coupling energy from the source to the load,four probes movable along the line and extending through the slot, meansresponsive to the wavelength of energy transmitted to the test load,means actuated by the wavelength responsive means for varying thespacings among the probes with changes in wavelength, the probes beingspaced apart an eighth wavelength or odd multiple thereof, and means forindicating changes in the difference between the squares of the voltagesat the first and third probes and at the second and fourth probes.

l2. Apparatus for measuring the impedance of a microwave load, saidapparatus comprising a source of microwave energy, means including asection of slotted line for coupling energy from the source to the load,a plurality of probes movable along the line and extending through theslot, means responsive to the wavelength of energy transmitted te thetest load, means actuated by the wavelength responsive means for varyingthe spacings among the probes with changes in wavelength, the probesbeing spaced apart an eighth wavelength or odd multiple thereof, meansfor detecting the output signals at said probes, and means forindicating variations in the relative potential as detected at each ofthe probes.

13. Apparatus for measuring the impedance of a microwave load, saidapparatus comprising a source of microwave energy, means including asection of slotted line for coupling energy from the source to the load,a plurality of probes movable along the line and extending through theslot, means responsive to the wavelength of energy transmitted to thetest load, means actuated by the wavelength responsive means for varyingthe spacings among the probes with changes in wavelength, means fordetecting the output signals at said probes, and means for indicatingvariations in the relative potential as detected at each of the probes.

14. Apparatus for providing a measure of the impedance of a microwaveload, said apparatus comprising a variable frequency source of microwaveenergy, means including a longitudinally slotted transmission line forcoupling energy from the source to the load, a series of n number ofmovable probes spaced along the transmission line and each projectinginto the longitudinal slot thereof, the number n being greater than two,means for determining the wavelength )t in the transmission line of thetransmitted microwave energy, means responsive to said last-named meansand varying the positions of the probes with variation in frequency ofthe source for maintaining the intervals between probes equal to an 2nwhere m is any odd integer, detector means coupled to each of theprobes, and means responsive to the output signals of each of saiddetector means, said last-named means including means for combining theoutput signals from the detector means vectorially, where the anglesbetween the vector representations of the output signals are degrees andthe magnitudes are proportional to the amplitudes of the output signals.

15. Apparatus as defined in claim 14 wherein nis equal to four and theprobes are spaced at an eighth wavelength or odd multiple thereof.

16. Apparatus as defined in claim 15 wherein said means for combiningthe outputs vectorially comprises means for taking the diiferencebetween the output signals as detected at the iirst and third of thefour probes and the difference between the second and fourth probes, thecombining means presenting the resultant of the difference signals addedvectorially in quadrature.

17. Apparatus for measuring the impedance of a microwave test load, saidapparatus comprising a variable frequency microwave energy source, iirstand second wave guide sections having longitudinal slots therein, meansfor coupling incident energy from the energy source into each of thewave guide sections, the first wave guide section being terminated in ashort circuit and the second wave guide section being terminated in thetest load, servo operated probe means associated with the iirst waveguide section, the probe means being automatically positioned along therst wave guide section in iixed relation to the detected standing wavepattern therealong, a plurality of movable probes extending through theslot in the second wave guide section, means for moving each of theprobes along the slot, means for maintaining equal spacings among theprobes, said means for moving each of the probes being actuated by saidservo operated probe means whereby the spacing between any twosuccessive probes along the second wave guide section varies withmovement of the probe means along the iirst wave guide section, andmeans for indicating variations in the relative potentials as detectedat the probes.

References Cited in the ile of this patent FOREIGN PATENTS 937,524France Mar. 8, 1948

